1. Field of the Invention
The present invention relates to a single-to-balanced band pass filter, and more particularly, to a single-to-balanced band pass filter prevented from disturbing by noises.
2. Description of the Prior Art
Please refer to FIG. 1, which illustrates a balanced-unbalanced converting circuit 21. The balanced-unbalanced converting circuit 21 includes capacitors C1, C2, C3, and C4, and strip lines 22, 23, 24, and 25. The strip lines 22, 23, 24, and 25 have terminals 22a, 22b, 23a, 23b, 24a, 24b, 25a, 25b. The capacitors C1, C2, C3, and C4 are all electrically connected to a ground G. There are also two balanced terminals 26 and 27, an unbalanced terminal 28, and an open terminal 29 in the balanced-unbalanced converting circuit 21. The balanced-unbalanced converting circuit 21 is used for transforming a balanced signal inputted from the balanced terminals 26 and 27, and thereby generating an unbalanced signal at the unbalanced terminal 28.
In the balanced-unbalanced converting circuit 21 shown in FIG. 1, noises may easily be introduced at the ground G and wrongfully effect operations of the balanced-unbalanced converting circuit 21 in forms of common-mode signals.